Storage tube compensation means



Oct. 31, 1961 A. s. I UFTMAN ET AL 3,007,078

STORAGE TUBE COMPENSATION MEANS Filed Aug. so, 195? SPIAL SWEET- VCJLTAGES United States Patent 3,007,078 STDRAGE TUBE COMPENSATION MEANS Alvin S. Luftman, Natick, Frank E. Taylor, Newton, and

John A. Buckbee, Wellesley, Mass., assignors to Raytheon Company, a corporation of Delaware Filed Aug. 30, 1957, Ser. No. 681,287 Claims. (Cl. 315-12) -This invention concerns a storage tube compens-ation circuit, and, more particularly, to means for varying the sensitivity or writing rate of a storage tube to compensate for variation in intensity of the presentation stored in the storage tube.

When storage tubes are written using a PPI radar scan, the central portions of the stored presentation are substantially overwritten as compared with regions near the edge of the stored presenta-tion. This results from the factthat the radial scan lines, which have physical width, converge in the center of the stored picture and overlap near the central portion, whereas these same radial lines may be spaced a considerable distance apart adjacent to the edge of the picture. It can be shown that if a uniform charge is deposited upon the storage surface during each PPI line scan and the scan is rotated at uniform speed, the charge pattern along a radius will be approximately parabolic.

Since storage tube overwriting can cause considerable deterioration off the stored picture from the standpoint of resolution and target coherence, it is desirable to introduce compensation for this overwriting eifect. In all storage tubes using secondary-emission writing near the rst cross-over of the dielectric secondary-emission curve, that is, at the critical potential or point other than zero storage screen potential at which the secondary emission ratio is unity, the writing rate is a function of the storage screen potential during the writing. By reducing the storage tube screen potential during writing toward the critical potential of the storage surface, the writing rate of the storage tube is reduced; similarly, an increase in the storage tube screen potential will increase the sensitivity or writing rate of the tube. In accordance with the invention, means are provided for compensating for the overwriting condition occurring during storage of video information in a storage tube by varying the storage screen potential during each radial scan in a controlled manner.

lt should be understood that the overwriting condition may also occur for scans other than PPI scans and the invention should not be restricted to use with PPI scanning systems.

The curve of writing speed versus storage screen voltage closely approximates a parabolic function within a useable range of screen voltage. As previously stated, the charge pattern along a radius of a PPI scan also approximates a parabolic distribution. Consequently, the desired correction may be obtained with sufficient accuracy by `applying a sawtooth voltage to the storage surface, rising in value from slightly above critical potential to a higher value of the order of 200 volts, depending in part upon the parameters of the individual tube. It may be shown that the sawtooth 4voltage can be set quite accurately for any storage tube by setting the value of storage screen voltage 1/16 of the radial distance from the center of the scan 3 'volts positive with respect to the critical potential for that storage tube and the value of voltage 5A; of the radial distance from lthe center of the scan at 25 volts positive relative to said critical potential.

The amount of overlap of the various scans is a function of both speed of rotation of the antenna and the pulse repetition rate (scan rate). Consequently, the amount of correction necessary is different, for example, When writing radar information at 600 pulses per second when the antenna is rotating at 5 r.p.m. than when writing 200 pulses per second when the antenna is rotating at 15 r.p.m.; in the first case, there are 7200 radial lines being written in one antenna rotation, While in the second case only 800 lines per antenna rotation are Written. To compensate for the varying amounts of information being written into the tube owing to variations in pulse repetition rate and antenna rotation speed, the amplitude of the video signal to the storage tube may be varied to compensate for varying amounts of information being written into the tube, thereby preventing overwriting conditions on the tube storage surface.

Further objects and advantages of this invention will become apparent as the description progresses, reference being had to the accompanying drawing wherein:

FIG. l is a circuit diagram showing a storage tube in cross section and associated circuitry for use therewith.

Referring to FIG. l, an image-recording storage tube 10 is shown, comprising an envelope, which may be made of glass, having an elongated neck portion 11 and enlarged portion 12 terminating in an end face 13. An electron gun 14, including a cathode 15, a control grid 16 containing an aperture, .a rst cylindrical anode 1=8 containing centrally located apertures at opposite ends thereof, and a second anode 19 are positioned within the neck portion 11 of tube 10. The second anode 19 may, for example, include an electrically-conductive coating on the inner surface of the tube, and may extend Well into the enlarged portion thereof. This coating may be connected directly to the lirst anode 18, as indicated in FIG. l. The necessary potential for the electron gun electrode, as well as for other electrodes of the storage tube, .are derived from a source of unidirectional energy, and practical values are indicated in FIG. l. It should be understood, of course, that these potential values are merely illustrative and that the invention is not limited to lany particular value of voltage. The storage screen voltage, of course, must be maintained at the proper value with respect to the critical potential in a manner to be pointed out subsequently.

Electrons emitted by cathode 15 are urged through the aperture of control grid 16 by the accelerating field between grid 16 and first anode 18. The electrons passing through the control grid aperture form a beam which, because of the electron lens comprising grid 16 and tirst anode 18, converges to -a first cross-over point on the longitudinal axis of the storage tube adjacent the frst aperture in anode 18. Beyond this cross-'over point, the beam starts to diverge.

Surrounding the neck of tube 11 are horizontal and vertical deflection coils, indicated by the single reference numeral 21, which are energized by appropriate sweep generators 23 and 25. Although the deflection system shown is electromagnetic, an electrostatic deflection system may be used, in which case, the ldeflection coils would be replaced by the standard perpendicular sets of deflection plates.

A magnetic focus coil 29 energized from an appropriate focus coil supply, not shown, surrounds the neck 11 of the storage tube in the region of the tube just beyond the accelerating anode 18. The focus coil 29 serves Ias a converging lens for focusing the electron beam on storage member 40, to be `described later. A collector or target electrode 32 is mounted adjacent to face 13 of tube 10 and serves as an electron collector during the reading operation, las will be explained more fully later. The collector electrode 32, in some instances, may comprise a fluorescent screen mounted on or near the face 13 of the tube or may consist of a lluorescent material applied directly to the inner surface of the tube face.

A cylindrical decelerating electrode 35 is positioned near the storage member 40 on the electron gun side of the storage member. Electrode l35 is maintained considerably more negative th-an accelerating anode 19 and, together with electrode .19, forms a decelerating lens owing to the retardation field between these electrodes; the electrons thereby are directed through the decelerating electrode 35 generally perpendicular thereto over a comparatively large deflection angle.

A storage member 40 is positioned within the enlarged portion of tube 10 between the decelerating electrode 35 and collector electrode 32 and generally parallel thereto. Storage member 40 consists' of :a dielectric storage material 42 coa-ted upon an electrically-conductive supporting mesh or screen-44. The storage l'ayer'42 is deposited preferably upon the-side of screen 44 facing the electron gun y14. Y

The operationof the storage tube may beY divided into three separate operations, namely, priming, writing and reading. An input signal containing the information to be written into the storage tube may be applied by' way of terminal -2 to control grid 16 when input switch 48 is in the position designated W and an unblanking pulse which is positive with respect to the cathode may be supplied to the control grid during the reading and priming operations, i.e., when input switch 48 is in the positions indicated as R and P, respectively. This unblanking signal overcomesthe cut-off bias on the grid 16 derived from battery 17. The amplitude of the input information Written into the tube is determined by the setting of potentiometer 54.

Before writing on the storage memberV 40, a priming operation-is achieved. When switch 27 is in position P, spiral sweep voltages are applied to deflection coils 21 and the electron beam is scanned over the elemental `areas of the storage surface 42 of ,storage member 40. The invention should not be restricted, however, to priming operations using spiral sweep voltages. When switch 58" is in the position designated' P a potential of the order of 2() volts positive with respect to cathode 15 is supplied to the electrically-conductive mesh 44 of the storage member 40. During the priming operation, the storage member potential, that is, the potential of the storage screen 44 relative to cathode 15, is kept below the critical voltage, that is, the voltage atv which the secondary emission ratio ofstorage surface 42 is unity. When the storage surface potential is less than this critical potential, which in typical tubes may be of the order of 50-volts, storage surface 42 Y will charge negatively until it reaches equilibrium at the cathode potential. The entire storage surface 42 scanned by the electron beam 22 thereby is uniformly charged to cathode potential. Switch 48 in the grid circuit of storage tube is in the position designated P while the storage surface is being primed negatively.

The Writing operation is achieved when the movable contacts of switches 27, 48, 58, yand 60 are moved-to the position indicatedA as W'. With input switch 48 in the position W, signal information at input terminal 52 is compensates for the number of radar pulses occurring per antenna rotation and for the number of lines written on fthe storage tube. During the writing operation, PPI sweep voltages are applied to sweep coils 21. It is necessary, when changing from the priming operation to the writing operation, to vary the potential applied to storage screen 44 so that this potential is above the cri-tical potential, rather than below the critical potential. 'I'he electron beam, as modulated bythe input signal, is carused to scan storage surface 42 and the latter 'is charged by the modulated beam current to .the storage screen potential. The amount of charge on each element of the tude Ywhen .that element is scanned.Y The chargegthatis formed on the elements of the storage surface thus is determined by the content of the input information. The invention should not be restricted to PPI deeotion systems for scanning during the writing period.

Durin-g the writing operation', the storage screen voltage is varied by means of a sweep generator shown in FIG. 1-. This generator may comprise an electron discharge device 64 including a cathode 67, a control grid 68 and ananode 69. A square wave voltage at terminal 61, which may be used also in connection with the generating of PPI sweep voltages, is supplied by way of coupling condenser 62 to the control grid 68 of tube `64. A grid bias resistor 71 is provided in the input circuit of the tube I64. The cathode 67 is connected to la negative potential of the order' of 1750 volts. The plate'69 of tube 64 is connected to B+- through a resistor 73 and a potentiometer 74 which includes a movable ,arm 75 and a resistive element 76 connected between two terminals which may be at +200 volts4v and volts, respectively, with respect to some reference potential, such as ground. The exact-values of po'- tential set forth are meant to be illustrative only andnot limiting values. A xed resistor 77, potentiometer 78 and capaci-tor 79 are connected in series between the plate 69 the voltage -across resistor 77 and potentiometerV 78 -andthe voltage across condenser 79. A trapezoidal wave form thereby is obtained consisting of a step whose value is determined by the voltage across resistive elements 77 and '78, that is, upon the setting of the potentiometer 78,

and a linear rising portion whose slope is determined not' only bythe time constant ofthe RC circuit, but also bythe plate voltage, which is a function ofthe setting of po-vV tentiometer 74 inthe plate circuit of tube 64. By adjust'-v m'ent of potentiometers 74 and 78,-it is possible to supply a trfapezoidal wave form (sawtooth wave form superim-4 posed upon a D.C. level) to storage member 40 .which increases linearly from some value, dependingV principally uponthe setting of potentiometer 7 8,` to ahigher value depending largely upon the setting of potentiometer 74. T hetwo .controls will have some minor eifect upon one an' other, butthis interaction is of a. second ordereffect. The

"storage screen potential may thus be Vvaried between'two limits, the lowe'rfone of which is close to, butnot lower than, the critical potential. These limits depend upon the overwriting condition above-mentioned; for example, the degree of variation overwrite from the center to the edge of the stored picture increases, the voltage range of the sawtooth` wave form lalso will be increased. The proper setting `of the potentiometer may be determined by optical examination of the stored picture.

When it is desiredto read the stored infornlation-outwhen switch 58 is the read position, where a potential of 4the orderl of +10 voltsV with respect to the cathode is indicated.

When an unmodulated electron beam is scanned across the storage elements, storage surface areas withv no charge will prevent thenelectron beam from reaching the oollecy Y When switch 27 is in the.

tor electrode,.while areas with charge values up to a said level will permit a portion of the electron beam correspoding to the amount of charge which has been written to reach the collector electrode. An amplitudemodulated current thus is received by the collector electrode corresponding with the charge pattern which previously. -has been written onto the storage member. This current ows through resistor 36 connected between the collector electrode 32 and electrode 35, land a voltage is obtained which is amplified in video ampliier 57 and connected to a video output terminal 59 by way of switch 60. The video output may 'be supplied to the intensity grid of a remote cathode ray tube, not shown, if desired.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. For example, although the use of a sawtooth voltage on the storage screen is suitable for use in PPI scans, where the amount of overlap of the various scans varies more or less uniformly from the beginning of each scan to the end of said scan, other types of scans will require the application of a diierent voltage wave form to the storage screen during each scan period. The shape of this voltage wave form on the storage screen will depend upon the characteristics of the scan and upon the number of said scans per unit time. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. In combination, a storage device having a storage screen adapted to be scanned in such a manner that the degree of overlap of successive scans tends to vary along the length of each scan to produce intensity Variations along said scan, means for Writing information into said storage device for a predetermined period, and means for varying the sensitivity of said storage device during the writing period to compensate for variations of intensity of each of said scans.

2. In a storage device having a storage screen adapted to be scanned in such a manner that the degree of overlap of successive scans tends to vary along the length of each scan, means for writing information into said storage device for a predetermined period, and means for varying the sensitivity of said storage device during the writing period inversely with the degree of overlap of each of said scans.

3. In a storage device having a storage screen adapted to be scanned in such a manner that the degree of overlap of successive scans tends to vary along the length of each scan, means for writing information into said storage device during each scan, the rate of writing being normally a function of said degree of overlap, and means for varying the sensitivity of said storage device during each of said scans to compensate for variations of writing rate over each of said scans.

4. In combination with a storage device having a storage element whose potential determines the sensitivity of said device, wherein an electron beam recurrently sweeps past said element, means for writing electrical information onto said storage element during each sweep at a Variable rate for constant storage element potential, and means for varying the potential of said storage element relative to a reference potential during each sweep to compensate for variations in writing rate.

5. In combination with a storage device having a storage element whose potential determines the sensitivity of said device, wherein an electron beam recurrently sweeps past said element, means for writing electrical information onto said storage element during each sweep yat a variable rate for constant storage element potential, means for varying the potential of said storage element relative to a reference potential during each sweep to compensate for variations in writing rate, and means for 6 varying lthe amplitude of said information in accordance with the characteristics of said information.

6. In combination with a cathode ray tube storage device having a storage element and an `intensity control grid wherein an electron beam recurrently sweeps past said element, means including said grid for writing electrical information onto said storage element during each sweep, and means for varying the potential of said storage element relative to a reference potential during each sweep to compensate for variations in writing rate caused by chan-ges in potential on said control grid.

7. In combination with a storage device having a storage element wherein an electron beam recurrently sweeps past said storage element, means for writing electrical information onto said storage element, said information normally causing variations in the rate of writing upon said storage element at different portions of said element, and means for varying the potential of said storage element during each sweep inversely -as the normal writing rate of said storage tube to compensate for said variations in writing rate upon said storage element.

8. In combination with a cathode ray tube storage device having a storage element and an intensity control grid wherein an electron beam recurrently sweeps radially from the center of said element outward `and simultaneously rotates about the center of said storage element, means including said grid for writing electrical information onto said storage element during each sweep, said information normally causing a greater degree of writing upon said storage element at the center of said element than at the edge, and means for Varying the potential of said storage element relative to a reference potential during each .sweep to compensate for said variations in writing rate caused by changes Iin potential on said control grid.

9. In combination with a cathode ray tube storage device having a storage element wherein an electron beam recurrently sweeps across said storage element, means for writing a charge pattern onto said storage element during each sweep which normally is dependent upon the electrical input information to be stored, said charge pattern being normally of varying intensity over dilerent portions of said storage element owing solely to peculiarities inherent in said sweep, and means for correcting during each sweep for said intensity variations by altering the storage element potential during each sweep from a value just above the critical potential of said storage element to a higher value dependent upon the peculiarities of said sweep.

10. In combination with a cathode ray tube storage device having a storage element wherein an electron beam recurrently sweeps -across said storage element, means for writing a charge pattern onto said storage element during each sweep which normally is dependent upon the electrical input information to be stored, said charge pattern being normally of varying intensity over different portions of said storage element owing solely to peculiarities inherent in said sweep, and means for correcting during each sweep for said intensity variations by linearly altering the storage element potential during each sweep from -a value just above the critical potential of said storage element to a higher value dependent upon the peculiarities of said sweep.

References Cited in the file of this patent UNITED STATES PATENTS 2,411,572 Hershberger Nov. 26, 1946 2,454,410 Snyder Nov. 23, 1948 2,456,952 Kluender Dec. 21, 1948 (Other references on following page) UNTEDsTAT'Es PATENTS VMesner Oct, 3, 1950 Russell et a1. Oct.V 10, 1950 Hergenrather Apr. 10,1951 Maynard et al. July 81952 Covely Sept, 20, 1955 

